Video images in a digital television system that meets the ATSC standard must be delivered in real time and with a consistent rate of presentation in order to preserve the illusion of motion. However, delays introduced by coding, multiplexing, and transmission can cause a variable amount of delay for video packets arriving at the decoder in the receiver. This delay can impair the decoding process in the receiver. Buffers in the decoder of the receiver are typically used as a mechanism to avoid the problems caused by this delay.
The MPEG-2 standard provides an additional mechanism to ensure video frames can be decoded and delivered to the viewer with a consistent rate of display. This additional mechanism is called the PCR or program clock reference. Values for the program clock reference are embedded into the adaptation field within the transport packets of defined PIDs in the MPEG bit stream. These PCR values provides a clock reference allowing the receiver to deliver a consistent display rate in spite of the variable delays with which the video packets arrive at the receiver.
However, because PCRs are used in the receiver to derive the clock reference, any jitter or drift in the PCR clock can have adverse effects on the receiver's performance. A major source of jitter in the PCR is PCR accuracy. This type of PCR jitter arises from a number of sources. For example, the repetition rate of the transport stream packets may not be a multiple of the PCR clock time, and the input and output data rates on the transport buffers may be poorly managed. Both of these sources are causes of PCR jitter.
One way to reduce the adverse effects of this jitter is to measure the jitter by comparing the PCR timestamps on the received transport stream with a local clock reference. The local reference clock is typically an integral multiple of the PCR clock. Therefore, any truncation and round-off errors in PCR jitter measurements are reduced or eliminated, thus enhancing the accuracy of the measurement. This measure of jitter is then used to adjust receiver decoding timing in order to present a consistent rate of presentation of the moving video images received by a digital television.
The present invention is directed to alternative or additional way of reducing the adverse effects of PCR jitter. According to the present invention, the PCRs are re-stamped in the transmitter in order to present the receiver with PCRs that reduce or eliminate jitter.